STRUCTURE OF THE UNIVERSE KAPTEYN. 



309 



Pig. 1. — Apparent Magnitude Boxes. 



ACCORDING TO MAGNITUDE AND PROPER MOTION. 



Second sets. Magnitude-motion boxes {fig. 2). 



Tlie star.s in each of the former series of boxes are redistributed 

 over a series of boxes, each of them containing stars of a determined 

 apparent motion. By way of an examj)le, figure 2 shows this new 

 classification for the stars of the fifth apparent magnitude. There 

 is, of course, another such series for each one of the apparent magni- 

 tudes. Those for the fifth have been distributed over 28 new boxes. 

 In the first have been collected the cards representing the stars with a 

 proper motion of 0" to 1" per century. The average motion is 0.5, 

 and this has been inscribed on the lid. The little arrow indicates that 

 this number represents a motion. The number 5 surrounded by a star 

 refers to the fact that we have exclusively to do with stars of the 

 fifth apparent magnitude. The second box contains the stars with 

 pro^Der motion between 1" and 2" ^iiv century, etc. For the larger 

 motions the limits have been taken somewhat wider. In the eleventh 

 box the motions 10" to 15" are contained; in the thirteenth, those 

 between 20" and 30" ; and so on. The nmnber of star cards in each 

 box has been inscribed on the lower right-hand corner of the lid. 

 The figure thus shows, for instance, that there are in the sky !)0 stars 

 of the fifth magnitude, having a proper motion between 0" and 1" per 

 century. We have thus arranged the stars according to both the 

 rough criteria of distance at our disposal. For we know j^erfectly 

 Avell that in a very general way the fainter the stars and the smaller 

 their apparent motion the farther they must be away. 



For each of the groups thus obtained we are now able, according to 

 what has been said before, to derive the mean distance. This deter- 

 mination being made, we obtain the mean distances expressed in 

 light years which have been inscribed on the lid with the letter 

 MD prefixed. 



